You don't need to with royal purple or most of the synthetics available. If you are getting a lot of noise on slow, tight turns, then you can add a bottle of ford friction modifier. It's not going to cause a vibration if it's not in there.
They may have not put the drive shaft back exactly how it was taken out. have them unbolt it and turn it 90 degrees. I also recommend torquing in a star pattern and sneaking up on the 41 ft lbs for the drive shaft to pinion flange bolts. I always torque mine in two stages. A shop probably just hits it with an impact gun.
Last edited by moosestang; 05-28-2013 at 01:50 PM.